Field of the Invention
This invention relates to a hydrostatic power unit in the form of a variable displacement machine with a variable, in particular a continuously variable, displacement volume and which can be operated as a pump and as a motor and is in a drive connection with an internal combustion engine. When the power unit is operated as a pump, it sucks hydraulic fluid on a suction side from a tank and delivers into a delivery side. When the power unit is operated as a motor, it is a hydraulic starter to start the internal combustion engine. When the power unit is operated as a motor, hydraulic fluid from a hydraulic accumulator is supplied to the suction side of the power unit. The displacement volume of the power unit can be varied by a displacement volume control device that can be actuated with a positioning piston device.
Description of Related Art
Hydrostatic power units which, when operated as a pump in which the power unit is driven by the running internal combustion engine, are used to charge a hydraulic accumulator and/or to supply at least one user with hydraulic fluid. When operated as a motor, the power unit is used as a hydraulic starter to start the shutoff internal combustion engine. Such power units are used in vehicles such as automobiles, trucks, or buses, and in mobile machines or construction equipment such as industrial trucks, agricultural equipment, forestry equipment, excavators, wheel loaders and telescoping loaders, tractors, combine harvesters, forage harvesters, and sugar beet or potato diggers.
To reduce fuel consumption when the internal combustion motor is running at idle, for example during waiting times, pauses or interruptions in work, a start-stop function is provided for the internal combustion engine. The internal combustion engine, which is running at no load, is shut off during pauses or interruptions in the work and is automatically restarted when torque is requested by a work function or by the traction drive. A rugged and reliable start-stop function of the internal combustion engine can be achieved with a hydraulic starter.
EP 2 308 795 A1 describes a hydrostatic power unit which, when operated as a pump in which the power unit is driven by an internal combustion engine and sucks hydraulic fluid with a suction side out of the tank, is used to supply a user of the working hydraulics with hydraulic fluid and functions by means of a charging circuit to charge a hydraulic accumulator. When operated as a motor, hydraulic fluid under pressure is delivered to the suction side of the power unit from the hydraulic accumulator and the power unit is driven by the hydraulic fluid from the hydraulic accumulator. The power unit operated as a motor generates an additional torque on the crankshaft of the internal combustion engine. The power unit operated as a motor can consequently be used as a hydraulic starter to start the shutoff internal combustion engine and/or as a booster drive to boost the running internal combustion engine.
If the power unit is a variable displacement machine with a continuously variable displacement volume, in operating the power unit as a motor for use as a hydraulic starter, the displacement volume control device of the power unit must be adjusted for startup of the shutoff internal combustion engine to increase the displacement, for example to a position with the maximum displacement, so that when the power unit is operated as a motor, the hydraulic fluid delivered from the hydraulic accumulator to the suction side of the power unit can generate a torque sufficient to start the internal combustion engine. Hydrostatic displacement machines, particularly variable displacement machines that are operated as pumps in an open circuit, are generally actuated by a spring device toward a minimum displacement volume so that the lowest possible drag torque is generated during a normal startup of the internal combustion engine by an electric starter.
On a variable displacement machine with a variable displacement volume, the displacement volume can be set (adjusted) by a displacement volume control device actuated by an actuator and control valve. The actuator and control valve is connected to a charging pressure circuit for the supply of hydraulic fluid. When the internal combustion engine is shut off, however, there is no charge pressure or volume flow available in the charging pressure circuit.
For adjusting of the power unit to start the internal combustion engine when the internal combustion engine is shut off, it is known from EP 2 308 795 A1 (FIG. 2) that a variable displacement device that controls the displacement volume of the power unit can be connected with the hydraulic accumulator so that when the internal combustion engine is shut off, the displacement volume control device of the power unit can be actuated starting from the position with minimum displacement volume and proceeding toward an increase of the displacement volume with the hydraulic fluid from the hydraulic accumulator to stroke the power unit for starting the internal combustion engine. The hydraulic accumulator is in communication via a pressure reducing valve with a charging pressure circuit which, when the internal combustion engine is running, supplies the variable displacement volume control device of the power unit with hydraulic fluid. With the pressure reducing valve, the pressure present in the hydraulic accumulator is reduced to the charge pressure level of the charging pressure circuit to adjust the displacement volume control device when the internal combustion engine is shut off.
The actuator and control valve of the displacement volume control device described in EP 2 308 795 A1 is designed for the charge pressure level of the charging pressure circuit in the low-pressure range of approximately 20 to 30 bar. In the charged hydraulic accumulator, however, there are pressures in the medium pressure and high pressure range of at least 100 bar. To be able to adjust the displacement volume of the power unit when the internal combustion is shut off with the hydraulic fluid from the hydraulic accumulator, an additional pressure reducing valve is therefore necessary to reduce the pressure level present in the hydraulic accumulator to the level of the charging pressure circuit for the supply of the actuator and control system of the power unit. However, a pressure reducing valve entails additional costs and takes up a good deal of space on account of its space-intensive construction.
If additional users other than the control and actuation system of the variable displacement machine to adjust the displacement volume control device when the internal combustion engine is shut off for a startup of the shutoff internal combustion engine, are to be supplied from the charging pressure circuit, a shutoff valve, such as a check valve, that cuts off the flow to the charging pressure circuit is required to prevent a discharge or outflow of the charging pressure to the other users of the charging pressure circuit and the tank generated from the hydraulic accumulator via the pressure reducing valve.
On account of the design of the actuator and control valve for the charge pressure level of the charging pressure circuit and, thus, the low-pressure level, a positioning piston device which is in an operative connection with the displacement volume control device for its actuation and control has a large pressurized piston surface to be able to generate the forces necessary for the actuation and control of the displacement volume control device Thus, there are large-volume positioning piston pressure chambers on the positioning piston device. To adjust the displacement volume control device of the power unit to the maximum displacement volume to start the shutoff internal combustion engine when the power unit is operated as a motor, relatively large quantities of hydraulic fluid are necessary, which are taken from the hydraulic accumulator to set the power unit to start the shutoff internal combustion engine. Because of this quantity of hydraulic fluid extracted from the hydraulic accumulator for the adjustment of the power unit, the pressure present in the hydraulic accumulator decreases so that the torque that can be delivered during operation of the power unit as a motor for the starting of the internal combustion engine, which is a function of the pressure present in the hydraulic accumulator, is reduced.
A further disadvantage of supplying the actuator and control valve by a pressure reducing valve from the pressure in the hydraulic accumulator is that the actuation time for the adjustment of the displacement volume of the power unit is directly proportional to the quantity of hydraulic fluid required by the positioning piston device and the hydraulic fluid volume flow from the hydraulic accumulator to the positioning piston pressure chamber of the positioning piston device. On account of the actuator and control valve and the positioning piston device designed for the charge pressure level, a corresponding amount of time is required during which, with the internal combustion engine shut off, the power unit is set to the maximum displacement volume, before the operation of the power unit as a motor can generate a torque sufficient to start the shutoff internal combustion engine, as a result of which the startup of the internal combustion is correspondingly delayed by the operation of the power unit as a motor, which can be subjectively unpleasant for a driver of the vehicle.
An object of the invention is to provide a hydrostatic power unit of the general type described above but which, with little added construction effort or expense, makes it possible to start the internal combustion engine with the power unit operating as a motor in a short period of time.